Telecommunication systems require optical fibre, which is capable of transmitting signals for a long distance without degradation. Such optical-fibre transmission systems often use single-mode optical fibres (SMFs), such as, for example, so-called “standard” single-mode fibres (SSMFs), which are used in terrestrial transmission systems.
To facilitate compatibility between optical systems from different manufacturers, the International Telecommunication Union (ITU) has defined several standards with which a standard optical transmission fibre should comply. Among these standards, the ITU-T G. 652 recommendation (November 2009) has several attributes (i.e. A, B, C and D) defining the fibre attributes of a single mode optical fibre with step-index profile. The ITU-T G. 657 recommendation focuses more precisely on bend insensitive single mode fibres.
As can be easily understood, loss is the key parameter for assessing the performance of such single-mode fibres, and any solution enabling reducing losses by 0.001 dB/km at the 1550 nm wavelength will significantly improve such single-mode fibres, as long as the other technical attributes and the manufacturing cost of the fibres are kept roughly the same. Actually, typical losses at the 1550 nm wavelength for a standard step-index single mode fibre are 0.19 dB/km, such that reducing losses by 0.001 dB/km would induce a −0.5% loss improvement.
It is well known that, for such single mode fibres, about 90% of the losses are due to the Rayleigh scattering at 1550 nm. The remaining 10% cover, on the one hand, losses induced by absorption mechanisms (OH-peak, Infrared losses, Ultraviolet losses) and, on the other hand, losses induced by an extra scattering mechanism, such as SAS (“Small Angle Scattering”) described in patent document U.S. Pat. No. 7,171,090.
It is also well known that sharp index profile transitions induce such extra scattering losses. As an example, G. 652 step index profile, which presents a core index step of about 5×10−3, induces extra scattering in the order of a few thousandth of dB/km (circa 0.005 dB/km) at the 1550 nm wavelength.
Actually, it must be recalled that an optical fiber is conventionally constituted of an optical core, which transmits an optical signal, and of an optical cladding, which confines the optical signal within the optical core. To that end the refractive index of the core, nc, is greater than the one of the cladding, nCl. An optical fiber is generally characterized by a refractive index profile that associates the refractive index (n) with the radius (r) of the optical fiber: the distance r with respect to the center of the optical fiber is shown on x-axis and the difference Δn between the refractive index at radius r, n(r), and the refractive index of the optical cladding nCl is shown on y-axis. Step index single mode fibres show a sharp index profile transition between the core and the cladding.
Designing core profiles having smoothed index transition has hence been investigated in the past, in an attempt to limit extra losses.
Patent document U.S. Pat. No. 7,171,090 hence describes an optical waveguide fibre designed with a soft transition of the index profile from the core to the cladding, which has low attenuation and low small angle scattering (SAS), at least in part due to a low-alpha profile (i.e., alpha less than 2.5). Actually, the low-alpha profiles produce a gradual change of refractive index, which contributes to decreased loss due to Small Angle Scattering (SAS).
Patent document U.S. Pat. No. 6,625,360 also describes single-mode optical fibres having an interfacial region between the core and the cladding with adequate index change, enabling achieving low loss phototransmission.
G. 652 single-mode fibres with alpha shape core profiles are also described in EP 2 369 379 and U.S. Pat. No. 7,876,990 patent documents. Although such an alpha profile allows a soft transition of the refractive index from the core to the cladding, it is not easy to control in the manufacturing process of a single mode fibre, such as a G. 652 SMF with a core diameter of circa 10 μm (hence much smaller than the core diameter of multimode fibres (50 μm or 62.5 μm), which often use alpha-graded index profiles).
It would hence be desirable to design a single mode optical fibre showing improvements over the prior art.
More precisely, it would be desirable to design a single mode optical fibre, which is easy to manufacture, and in which scattering losses due to index changes are reduced, without degrading Rayleigh scattering.